This invention relates to the reduction of scale in the polymerization of vinyl monomers. In particular, it relates to the use of an acid hydrazone condensate or a salt thereof for that purpose.
During the polymerization of vinyl monomers, such as polyvinyl choride (PVC), tenaciously-adhering polymer scales frequently form on reactor components that come into contact with the monomer, such as the inner walls and stirrer. The reactor has to be cleaned between each batch by scraping, pressure-washing, or solvent cleaning. As a result, there is a considerable amount of reactor downtime as well as increased costs associated with maintaining a polymerization-ready reactor, which increases the overall operating cost of the process. Moreover, an operator who attempts to physically remove the scales risks exposure to harmful monomer trapped in the scales. Scale deposition also lowers polymer yield, reduces heat transfer efficiency during heating and cooling, and lowers monomer efficiency and general process control. When scales flake off and mix with the polymer, the polymer has poorer processing properties and finished products made from it are inferior. A number of different additives are known that reduce scaling, but many of them increase the yellowness of the polymer, which is unacceptable to many customers.